1. Field of the Invention
This invention relates to synthetic resin-coated springs which have a good self-silencing property or damping effect and a good corrosion resistance, and also to a method for making such springs as mentioned above.
2. Description of the Prior Art
Springs have heretofore widely been used as an industrial part in the fields of automobiles and other vehicles. Among such springs, there are coil springs which are used in a suspension system between the axle and the body of automobiles. The coil springs serve as a cushion between the axle and the body and function as an important part in the automobile. These conventional coil springs raise a problem that they will collide with each other when suffering vibrations, shocks and the like produced during the travel of automobiles thereby to make a clattering noise.
Moreover, the coil springs tend to undergo corrosion by the action of moisture or an electrolytic solution, such as a road antifreezing agent, with an attendant problem that there is the fear of spring breakage caused by corrosion fatigue.
In order to solve the above problems, there have been used coil springs wound with a polyurethane sheet or sheathed in a rubber or resin tube. However, the winding or sheating is conducted by hands, so that the operation is complicated and is not economical. In addition, the adhesion of the sheet or tube to the coil spring is not satisfactory, thus making it difficult to effectively prevent the penetration of water into between the sheet or tube and the spring. On the other hand, there have been proposed methods comprising coating coil springs with resins in, for example, Japanese Laid-open Patent Application Nos. 57-69123 and 57-136971, and methods comprising using adhesives and/or sealing agents between such a sheet or tube and the spring. However, these coil springs are poor in durability under severe service conditions where they are subjected to repeated stress. Thus, good results cannot be expected.